1. Field of the Invention
The present invention relates to an image processing device that executes image processing for matching two images, and an image processing program storage medium that stores an image processing program for causing a computer to operate as the image processing device.
2. Description of the Related Art
In the medical field, medical images obtained by photographing the body of a subject by means of X-ray and the like are widely used for diagnosing the condition of the subject. Use of medical images for diagnosis makes it possible to grasp the development of the condition of a subject without giving external damages to the subject, and to readily obtain the information required to decide a treatment plan.
Also, in recent years, there have been widely used devices such as Computed Radiography (CR) device that obtains digital medical images by using X-ray, and Computerized Tomography (CT) device that obtains a tomogram image of a subject by using X-ray, and Magnetic Resonance Imaging (MRI) device that obtains a tomogram image of a subject by using high magnetic field. Accordingly, instead of conventional medical images using X-ray films, digital medical images are generally used.
Because medical images are digitized, it is possible to apply image processing to medical images, thereby performing diagnosis more effectively than conventional ways. As one of such image processing to be applied to medical images, attention has been focused on image processing called Energy Subtraction (ES) processing in recent years. In the ES processing, from a pixel value in one of two images photographed by using two kinds of X-rays having different energies, a pixel value in the other is subtracted after being assigned a certain weight, so that among body parts forming the body structure of a subject in the former medical image, an image of a body part having a certain absorption property with respect to X-rays can be erased. According to the ES processing, it is possible to obtain a soft-part image formed only by soft-tissue images by erasing bone images from a medial image, and on the contrary, to obtain a bone-part image formed only by bone images by erasing soft-tissue images from a medial image. As a result, a focus hidden by a bone, a focus within a bone, or the like can be readily discovered and thus effective diagnosis can be performed.
Here, it is desirable that two medical images match each other so that the ES processing is performed with high accuracy. In late years, it is widely adopted photography using the so-called flat panel detector (FPD) in the field of X-ray photography, in which X-ray detecting elements are two dimensionally arrayed. In this type of detector however, a single medical image is obtained by single photography and thus, the photography times of two medical images are different. Therefore, it is very likely that two medical images disagree due to the attitude or respiration of a subject at each photography time. If the ES processing is forcibly applied to two medical images that are disagree, a virtual image (artifact) that did not originally exist appears in a displacement between the two medical images, which obstructs the reading of the images.
In this regard, there is proposed a technique for applying image processing to two medical images to be subjected to the ES processing and the like, such that one of the these two images is transformed to agree with the other (see, for example, Japanese Patent Application Publication No. 2002-32735, No. 2002-32764 and No. 2003-244542).
Incidentally, as one of representative medical images, there is a chest image obtained by photographing a chest part of a subject. The chest image shows a heart whose shape changes by heartbeat. The movement of a heart is different from those of other elements such as lungs, blood vessels and bones in terms of direction, amount or the like of movement. As a result, there are cases in which even if, for example, image processing disclosed by any of the above-mentioned patent application publications is applied to two chest images, a displacement remains in a heart portion or the periphery thereof and thus an artifact occurs after the ES processing.